Magnetoresistive random access memory (MRAM) is a type of memory device containing an array of MRAM cells that store data using their resistance values instead of electronic charges. Generally, each MRAM cell includes a magnetic tunnel junction (MTJ) structure. The MTJ structure may have adjustable resistance to represent a logic state “0” or “1.” The MTJ structure typically includes a stack of magnetic layers having a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric, e.g., an insulating tunneling layer. A top and a bottom electrode are utilized to sandwich the MTJ structure so electric current may flow between the top and the bottom electrode.
One ferromagnetic layer, e.g., a reference layer, is characterized by a magnetization with a fixed direction. The other ferromagnetic layer, e.g., a storage layer, is characterized by a magnetization with a direction that is varied upon writing of the device, such as by applying a magnetic field. When the respective magnetizations of the reference layer and the storage layer are antiparallel, a resistance of the magnetic tunnel junction is high having a resistance value Rmax corresponding to a high logic state “1”. On the other hand, when the respective magnetizations are parallel, the resistance of the magnetic tunnel junction is low, namely having a resistance value Rmin corresponding to a low logic state “0”. A logic state of a MRAM cell is read by comparing its resistance value to a reference resistance value Rref, which is derived from a reference cell or a group of reference cells and represents an in-between resistance value between that of the high logic state “1” and the low logic state “0”.
A MRAM device is conventionally manufactured by photolithography, in which silicon oxide and tin nitride is used as a mask patterning the stack of magnetic layers including the ferromagnetic layers and the insulating tunneling layer. The hard mask layers disposed on the stack of magnetic layers are patterned to form an array of dots. Portions of the stack of magnetic layers exposed by the array of dots are then etched away to form a corresponding array of MRAM cells. Subsequently, the hard mask layers stripped to result in a MRAM device.
As the size of MRAM cells decreases, conventional etching process as performed to pattern the stack of magnetic layers become problematic. For example, conventional etching processes often generate solid etching by-products which may redeposit onto the sidewall of the MTJ structure during the patterning process. As the size of MRAM cells decrease, redepositing of the etching by-products may create undesired etching profile, e.g., sloped, slant or tapered sidewall or early close-up of the features as etched, thereby leading to critical dimension inaccurate and circuit shorting. Furthermore, etching by-product built-up may also result in incomplete etching of the MTJ structures, thereby resulting is connected pitch patterns at the bottom of the MTJ structures, eventually leading to device failure.
Therefore, there is a need in the art for improved methods and apparatus for fabricating MTJ structures for MRAM applications.